1. Field of the Related Invention
The present invention relates to a method for molding hollow shaped bodies such as assist grips, door grips, and molding strips.
2. Description of the Related Art
An assist grip, such as one schematically shown in FIG. 20, indicated with reference numeral 9, for use in the interior of automobiles is generally required to be lightweight, and is therefore made as a hollow shaped body. The assist grip comprises a synthetic-resin made skin 93 and a hollow portion 95 therein.
In the beginning of molding the hollow shaped body described above, as shown in FIG. 21, a molten synthetic resin 90 is injected into a mold cavity 80 provided inside the mold block 8 at an amount sufficient to form the skin, i.e., the outer wall, of the hollow shaped body. Accordingly, a completed injection leaves an unfilled portion 801 remaining uncharged with the synthetic resin 90 between the front end 901 of the injected resin and a wall of the cavity 80. This is the so-called "short-shot" state.
Immediately thereafter, in the next step as shown in FIG. 22, to the inside of the synthetic resin 90 at the short-shot state is introduced a gas 7 such as nitrogen and air to thereby form a hollow portion therein. After introducing the gas 7, a little amount of molten resin 90 is further injected to the mold cavity. The introducing of the gas 7 is continued until the front end 901 of the resin completely fills in the aforementioned unfilled portion 801 which remained uncharged with the resin. Cooling comes next, upon completion of the gas introducing step.
Thus, a hollow portion 95 is formed inside the synthetic resin 90 by introducing the gas 7 inside, and on the outside thereof is formed the skin 93. That is, a hollow shaped body comprising a skin 93 having a hollow portion 95 therein is formed as disclosed in JP-B Nos. 57-14968 and 54-15291, and JP-A No. 53-9870 ("JP-B" as used herein stands for "published examined Japanese Patent Application", and "JP-A" for "published unexamined Japanese Patent Application").
To the outer surface of the skin 93 is transferred, further, a grained pattern or the like provided on the inside wall of the cavity.
In FIGS. 21 and 22, the mold block 8 aforementioned comprises a plate 81 for fixing the metal mold at the fixed side, a pusher plate 82, a fixed block 83, and a movable block 84. Between the fixed block 83 and the movable block 84 is provided the mold cavity 80 inside which is formed the hollow shaped body.
Further to the metal-mold attachment plate 81 is provided a sprue bush 85 and a Z-shaped sprue-puller pin 86. The sprue bush 85 comprises a sprue 851 through which the synthetic resin 90 and the gas 7 is injected. The sprue-puller pin 86 comprises a Z-shaped cutout at the front edge, and is necessary for drawing out the excess resin remaining inside a runner 831. The fixed block 83 comprises a runner 831 and a gate 835.
The foregoing method, however, presents the following problems.
The FIG. 20 assist grip 9 which can be attached to the interior of an automobile via the hole 96, was molded by a conventional method for molding a hollow shaped body. Such hollow shaped bodies, however, suffer from the formation of a hesitation mark 91 on the surface. This hesitation mark, which is also known as uneven luster, is unfavorable because it renders poor appearance to the product.
The inventors, after an intensive study on the generation of such unwanted hesitation mark 91, found that the generation thereof was ascribed to the process comprising a temporary stop at the injection of the molten synthetic resin 90 in the short shot state (as shown in FIG. 21) followed by incorporating thereafter the gas 7.
That is, such a stop, though only a short period of time, is sufficient for cooling the front end 901 of the molten synthetic resin. Accordingly, even if the gas is incorporated thereafter to extend the front end 901 by the action of the gas, the cooled front end would not intimately make contact with the inner wall of the cavity 8, and thereby the pattern, such as crimps provided to the inner wall of the cavity, would be insufficiently transferred. This means that a plane surface would be provided to the front end 901 and thus the hesitation mark 91 described above is produced. It can be seen, further, that the hesitation mark 91 is formed locally on the front end 901 of the synthetic resin which stayed in the short shot state (see FIG. 21, shown with arrow A).
The aforementioned hollow shaped bodies further occasionally suffers dents, i.e., shrink marks, which are believed to form during molding, by lack of balance in shrinking and pressure ascribed to the non-uniform thickness of the wall of the hollow portion.